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* specify chunks for pipe/transformer benchmark

Set chunks to be equal to len(balance) for pipe/transformer benchmark. Will update words per second and memory usage checks in next commit (must test on CircleCI to find appropriate values)

* change benchmark words per second and memory usage

Did six runs for words-per-second, with results: 9144.40, 9163.91, 9993.01, 9082.82, 9155.09, 9000.67
Peak allocated bytes per device (which does not change between runs) were 193206272, 645632, 562688, 92688384 for devices 0, 1, 2 and 3, respectively

* increase batch size

batch size was small enough that the GPU's computing power was not the bottleneck, slowing training and specifically making more chunks slower. Increasing batch size has therefore increased training speed

* update benchmark numbers

ran six times, with wps 36917.44, 36797.65, 37006.03, 36872.84, 37129.31, 37003.31 and peak allocated bytes 4061909504, 4050944, 10427392, 2031824896 for devices 0,1,2 and 3 respectively.
Co-authored-by: Jun Ru Anderson <andersonic@fb.com>